This invention relates to tufting machine hooks and more particularly to such hooks used in conjunction with knives for forming cut pile tufted fabric.
In the production of tufted pile fabric each reciprocating needle cooperates with a looper or hook which seizes a loop of yarn from the needle and releases the loop to form loop pile fabric or holds the loop until it is cut by a knife acting in scissors-like fashion against the side of the hook to form cut pile fabric. The cutting action occurs as the hook and knife oscillate along different paths, each being retained in a respective support. Conventionally, the knives are mounted in knife blocks including a cylindrical peg extending from the body thereof for insertion within a bore formed in a knife bar as illustrated, for example, in U.S. Pat. No. 3,084,645. Since the knives must act against the face of the respective hooks in scissors-like fashion with sufficient pressure, the knife block is pivoted within the bore to engage the face of the hook at an angle of inclination, generally in the order of approximately 8.degree., and contact the hook at an angle of canter, generally approximately 4.degree.. The combination of the angle of inclination and the angle of canter provides a compound angle between the knives and the respective hooks. To obtain proper cutting action, the pressure or tension of the knife against the hook must be sufficient to obtain the scissors-type cutting action. To provide the proper tension or loading of the knife against the hook the knife bar is adjusted so that the knife actually flexes or bends due to the loading or forces between the knife and the hook.
However, there are limitations on the loading of the knife since cutting occurs at a location adjacent to the throat of the hook and if contact occurs between the surface of the knife below the cutting edge and the surface of the hook behind the throat, the knife is said to be "heeling" against the hook and cutting either ceases or a very rough cut occurs. Moreover, tension on the knife may be limited in very fine gauge tufting machines, i.e., where the gauge is one-tenth or less since a knife may contact an adjacent hook. In such fine gauge machines a knife relief is often ground into the surface of the hook remote from the cutting surface to prevent or minimize such interference. Consequently, although high knife tension is desirable it is difficult to attain with conventional hooks. In such fine gauge machines the thickness of the yarns that may be cut is limited because of the limitations on the tension that can be applied between the knives and the hooks either due to the "heeling" affect or the limitations on the hook geometry.
Moreover, since the knives and hooks continuously contact one another hook and knife wear occurs rapidly. Once the cutting edges are worn proper cutting action ceases, resulting in very rough cut yarn ends.
Another problem encountered during cutting in a tufting machine results from the fact that the knives act against one face of the hook, and since the hook has a thickness, the leg of the yarn cut against the cutting surface of the hook is shorter than the other leg by an amount substantially equal to the thickness of the hook. Thus, the pile fabric produced has unequal length ends projecting from the backing material and requires substantial tip shearing.